1. Field of the Invention
This invention relates to a solid substance removing device, a purifying device, a method for the removal of solid substance, and a method for the production of (meth)acrylic acid.
2. Description of Related Art
Such easily polymerizing compounds as acrylic acid and methacrylic acid are raw materials for commercial production and are chemical substances which are produced in a large amount at a plant of a large scale. In the case of (meth)acrylic acid, for example, the easily polymerizing compound is produced by the reaction of catalytic gas phase oxidation of propylene, isobutylene, t-butanol, methyl-t-butyl ether, or acrolein. The reaction gas obtained by the reaction of catalytic gas phase oxidation contains (meth)acrylic acid, the target product, as mixed with other by-products. For example, this reaction generates mainly non-condensable gases, namely unreacted propylene and isobutylene; low-boiling compounds having lower boiling points than acrylic acid, namely steam and unreacted acrolein, formaldehyde produced by a side reaction, and impurities such as acetic acid; and high-boiling compounds having higher boiling points than acrylic acid, namely maleic anhydride, furfural, benzaldehyde, benzoic acid, and acrylic acid dimer. For the purpose of purifying this reaction gas thereby obtaining the target product, therefore, the reaction gas is washed with a counter flow water or heavy solvent thereby extracting the gas and subsequently supplying the extracted gas to a purifying column and completing the target product through various means such as distillation, stripping, absorption, and purification generally.
The easily polymerizing substances, however, have such a chemical property that they are possibly compelled by the heating conditions or the pressure conditions prevailing during the process of production to form polymers and gelling matter under the influence of impurities and by-products.
When such solid substance impurities are not extracted as a target product but are suffered to stagnate in the fluid under treatment or adhere thereto and deposit thereon, they possibly entail further adhesion of other impurities thereto. The solid substance which stagnates over, adheres to, or deposits on the interior of a purifying column clog pipes and devices attached to the purifying column. Otherwise, the solid substance partly mingles into the target product and causes a degradation of the quality of the product. For the purpose of stably operating the purifying column by maintaining the quality of treatment in the column, therefore, it becomes necessary to operate the periodic observation of opening the purifying column and removing the polymer adhering to the inner wall of the column. This extra work greatly consumes time and labor and profusely degrades the productivity.
As a means for solving these problems, a method which consists in attaching a filtering device to a strainer or a filter of the purifying column has been conceived. Such a known removing device, however, is incapable of thoroughly removing a solid substance impurity such as precipitate or polymer. When the solid substance impurity which has escaped the removal adheres to the interior of a device of the subsequent step, as a result the process can not operate sequentially because the removing device has no alternative but to cease operating. For the purpose of enabling the purifying column to operate stably by maintaining the quality of the fluid under treatment therein, it is necessary to remove periodically the polymer adhering to the inner wall of the purifying column with an enormous cost of time and labor and at the sacrifice of productivity of the target product, contrary to the object of mass production.
The official gazette of JP-A-08-239341, for example, discloses a method for separating oligomer, etc. by extracting from a purifying column the reflux liquid descending the interior of the column from at least one point of the column and separating the (meth)acrylic acid of the oligomer and/or the polymer present in the extracted reflux liquid. Specifically, as concrete examples of the means for separating the (meth)acrylic acid of the oligomer and/or the polymer from the refluxed liquid mentioned above, methods which resort to ultrafiltration, and chromatography and a method which utilizes a temporary retaining tank enabling the fluid of a distilling device, for example, to remain in the boiling state are enumerated.
When the purifying column is started by the conventional method and the object for purification is such an easily polymerizing substance as (meth)acrylic acid, the places which are liable to generate a polymer include the bottom part of the wall of the purifying column, the condenser attached to the purifying column, and the reboiler requiring temperature elevation, for example. When a strainer is attached to the extraction pump connected to the bottom of the column, therefore, the pump is not only clogged by the adhesion of the polymer but also stopped by the occurrence of cavitation due to adhesion of the polymer to the interior of the strainer. Thus, the safe operation of the purifying column is possibly spoiled.
There is also a time when the stagnation of a liquid generates a polymer, gradually accumulates as a core of the polymer, and induces further polymerization and clogging.
Further, the adhesion of the polymer to the strainer or the clogging of the strainer with the polymer necessitates a cleaning operation. The chemical stimulation by the raw material for the reaction, the reaction product, and the by-products and the physical hindrances due to the adhesion of a polymer inflict a spiritual displeasure on the worker engaged in the cleaning operation and bring about an alarming influence on his health. Further, the cleaning work entails the problem of jeopardizing his safety as the n of an organic solvent to be used for distillation. Further, the manual removal of the waste of the washing work for disposal is generally difficult, based on the size of the device used for the washing and removing work and the size of the purifying column itself as contributory factors.
The present inventor, after pursuing an elaborate study on the solid substance which is generated in a purifying column handling an easily polymerizing substance, has found that the solid substance can be easily removed by the use of a solid substance removing device attached to a specific filtering part and further occurrence of a polymer can be prevented by having the device incorporated in the process for the production of an easily polymerizing substance. This invention has been perfected as a result. The tasks mentioned above are accomplished by the following items (1)-(4).
(1) A solid substance removing device comprising a body provided with a fluid inlet pipe and a fluid outlet pipe and a filtering part disposed inside the body between the fluid inlet pipe and the fluid outlet pipe inside the body, the filtering part having (a) a thickness in the range of 5xcx9c500 mm, (b) a percentage of voids in the range of 60xcx9c99.5 vol. %, and (c) a contact surface area in the range of 100xcx9c2000 m2/m3.
(2) A purifying apparatus which has disposed in a purifying column handling an easily polymerizing substance and provided with a solid substance removing device thereon set forth in (1) above.
(3) A method for the removal of a solid substance, characterized by introducing the bottom liquid of a purifying column handling an easily polymerizing substance into a solid substance removing device set forth in (1) above and circulating at least part of the fluid discharged through the fluid outlet pipe of the device to the purifying column.
(4) A method for the production of (meth)acrylic acid or an ester thereof by the use of a method set forth in (3) above.
According to this invention, a solid substance removing device which permits removal of solid substance in a purifying column particularly handling an easily polymerizing substance, prevents new generation of a polymer, and causes a polymer to transfer to the next step only sparingly is provided. The solid substance removing device of this invention, when disposed in a purifying column, is enabled to remove the polymer in the purifying column. By using the solid substance removing device and the purifying apparatus according to this invention, it is made possible to produce an easily polymerizing substance very stably for a long time.